Same Class II hood for different species

Having heard many different views about this over the years, I'd like to get other external (and hopefully expert ) opinions. The consensus is that primary human cells and immortalized cell lines should be handled in separate dedicated hoods, but what are your thoughts on using one Class II hood for both animal and human primary cultures? Obviously equipment availability is a limiting factor but could this mixed use lead to any problems? i have a colleague who is adamant they shouldn't be handled in the same hood and ideally, should be in separate rooms but they couldn't tell me why. They were taught to keep everything separate but not taught the reasons why. Looking forward to hearing everyone's thoughts on this.

With proper technique there is no reason to keep either primary lines and immortal lines separate or keep different species separate. The only reason why you might want to do either is to eliminate the possibility of cross-contamination (approx 30-50% of immortal cell lines are actually HeLa variants due to cross-contamination). If you are particularly worried about cross-contamination (as it seems your lab is) then something as simple as keeping separate media for each line and using filtered pipettes should be enough to prevent the contamination, even just physical time between working with cell lines should be enough (i.e. work with one line, wipe hood etc. down, work with next line...)

Think about it like this... How often do you get a bacterial (viral, yeast) contamination of your cultures? I'm guessing not often, even if you don't use antibiotics, bacteria are far far smaller than a eukaryotic cell and far far more likely to spread via aerosols - so how big do you think the chances of contaminating your work with another cell line are?

The reasons for cross contamination with HeLa are largely due to poor technique used in the early days of cell culture, before hoods were invented, and when people worked on benches, often not changing pipettes between lines and things like that.